MORPHOLOGY 73 



and numerous nuclei. Their sap is a milky, usually white fluid, 

 which contains gum-resins, i.e. mixtures of gums and resins, 

 caoutchouc, fat and wax in emulsion. In addition, they sometimes 

 hold in solution enzymes, leptomin, tannins, often poisonous 

 alkaloids, and salts, especially calcium malate, also in the case of 

 Ficus Carica and Carica Papaya peptonising ferments. Proteid 

 granules often occur in the latex, and in the latex cells of the 

 Euphorbiaceae there are also present peculiar dumb-bell-shaped starch 

 grains. On exposure to the air the milky sap quickly coagulates. 

 In the adjoining figure (Fig. 78 D) is shown a portion of an isolated 

 latex cell dissected out of the stem of an Asclepiadaceous plant, 

 Ceropegia stapelioides. 



Special cells, which differ in form, contents, or in their peculiar 

 wall thickenings from their neighbouring cells, are distinguished as 

 IDIOBLASTS. If strongly thickened and lignified, they are called 

 sclerotic cells (stone cells) or sclereides. They often contain ferments; 

 in the Cruciferae and some other orders myrosin is thus present, 

 Avhile Prunus laurocerasus contains emulsin. For the most part they 

 contain excreted substances such as tannins and calcium oxalate. 

 In Fig. 85 an idioblast, containing a bundle of raphides, is represented. 

 Idioblasts, resembling tracheides and functioning as water reservoirs, 

 are found between the chlorophyll-containing cells in the leaves of 

 some of the Orchidaceae and Cactaceae. 



Size of Cells. The corresponding cells of equivalent members 

 of the same plant are usually of nearly the same size, even when the 

 members show a variation in size. 



Differentiation of the Protoplasts. In organisms composed 

 of one or of few cells the separate parts of the same protoplasts 

 may under some circumstances exhibit marked differences. On the 

 other hand, in multicellular organisms whole protoplasts are specialised 

 for functions with the performance of which their definite structure 

 is connected. 



Inclusions of the Protoplasm STARCH. The chloroplasts in 

 plants exposed to the light almost always contain starch grains. 

 These grains of starch found in the chloroplasts are the first visible 

 products of the assimilation of inorganic matter. They are formed 

 in large numbers, but as they are continually dissolving, always 

 remain small. Large starch grains are found only in the reservoirs 

 of reserve material, where starch is formed from the deposited 

 products of previous assimilation. Such starch is termed RESERVE 

 STARCH, in contrast to the ASSIMILATION STARCH formed in the 

 chloroplasts. All starch used for economic purposes is reserve 

 starch. The starch grains stored as reserve material in potatoes are 

 comparatively large, attaining an average size of 0'09 mm. As 

 shown in the following figure (Fig. 79), they are plainly stratified. 

 The stratification is due to the varying densities of the successive 



